1) Field of the Invention
This invention relates to a process for producing porous membranes which are useful, for example, in the filtration of fluids to remove any fine particles present therein.
2) Description of the Prior Art
Porous membranes are being utilized as filtering membranes and separating membranes in various fields of industry, and there is a continuing demand for porous membranes having a narrow pore diameter distribution in order to improve their performance characteristics in the removal and fractionation of fine particles present in fluids. In order to meet this demand, porous membranes of the straight pore type are now available. These porous membranes are made by bonbarding a polymer film of polycarbonate or the like with charged particles to form cylindrical pores extending in the direction of the film thickness.
Also known is a wet coagulation process for producing porous membranes from a polymer solution.
However, in the process for producing porous membranes by bombardment with charged particles, it is difficult to enhance the surface pore density of the porous membrane. In fact, conventional porous membranes produced by this process have a surface pore density of not greater than about 20%. Accordingly, these porous membranes have the disadvantage that such a low surface pore density fails to give a satisfactorily high fluid permeability per unit area of membrane.
Moreover, this process involves the use of a nuclear reactor for the generation of charged particles. The need for such a large-scale apparatus is another disadvantage of this process.
In the wet coagulation process using a coagulation bath, a nonporous skin layer tends to be formed on the surface of the porous membrane, thus making it difficult to enhance its permeability to fluids. Moreover, the use of a coagulation bath has the disadvantages of increasing the size of the apparatus, requiring the adjustment of its concentration, and raising the cost of recovery of the solvent.